You want wasabi with that?

When Hari Kunzru started rustling up seaweed salads and sashimi at home, he found that Japanese cookery didn't live up to its forbidding reputation. Here, he indulges his growing obsession with a masterclass from top sushi chef, Nic Watt

Novelist Hari Kunzru having a sushi masterclass from chef Nic Watt at Roka, a Japanese restaurant in Bloomsbury, London. Photograph: Sarah Lee

I still remember the first time I ate Japanese food, on a student date with a worldly girlfriend who was appalled that I had never tried it before. I instantly fell in love with the clean simple tastes, the aesthetics of the present-ation. Ever since then I have sought out good Japanese food - not an easy thing in the UK, where glutinous conveyor-belt sushi and salty soup-noodles still rule most restaurant tables. Some of the best meals in my life have been Japanese - the hole-in-the-wall place in Seattle where the chef served us savoury chawanmushi egg custard laced with little slivers of sea urchin, the Kaiseki Ryori banquet in Tokyo where I ate steamed snow-crab and grilled sardine with pickled plums and blowfish soup and about 20 other beautiful things I only remember because I kept a translation of the menu.

More recently I have started to cook Japanese food at home. Contrary to its forbidding reputation, it is actually very easy - at least to grasp the basics. Much of it is simply about having the ingredients to hand. If your cupboard contains rice, miso paste, soy, sweet mirin rice wine, sesame seeds and a few other bits and pieces, you can quickly make a wide range of dishes. There are a lot of Vietnamese shops in my neighbourhood in London and I can pick up some ingredients locally. For others, I have to go into Soho, where there is a Japanese supermarket, always filled with homesick students desperate for a taste of home. (If there are no Japanese food stockists near you buy online at www.mountfuji.co.uk). I make seaweed salads and cold soba noodles and (on the rare occasions when I can drag myself out of bed and get to Billingsgate fish market), I indulge in the luxury of carving slices of sashimi off a block of tuna and eating it for breakfast, over the papers, with a cup of green tea.

But there is only so far you can go without help, and Japanese food culture is as rich and complex as any in the world. So it was a pleasure to spend an afternoon with Nic Watt, a New Zealander who has made it his life's work. Watt has cooked at globally renowned places such as Nobu and the Kozue restaurant at the Tokyo Park Hyatt. He is now head chef at Roka, in Charlotte Street, Soho, a restaurant based on the Japanese robata tradition of boozy, charcoal-grilled dining. In its London incarnation that means media lunch a go-go and when I arrive there one Friday afternoon, the place is still humming with advertising execs hammering their expense accounts, wind-assisted by the huge range of shochu (think flavoured vodka, only classy) available from the bar downstairs.

We start with a little sushi-rolling, a series of formal little gestures for which I have no natural aptitude but plenty of enthusiasm. Wet your hands before you handle the rice, clap once to remove the excess water. Scoop and shape palmfuls of sticky white grains, working them out across a sheet of crispy nori seaweed. Line it with your fillings, fold the lot into a rolling mat and cut.

Knives are treated with a great, almost fetishistic reverence. We are using hand-forged steel blades shaped like long, thin leaves, stored in little wooden scabbards, to leach away moisture. They need to be wiped from the back to avoid amputating your fingers and when you wet them before cutting, you stand them briefly on their ends, to let droplets roll down the cutting edge, a moment I feel ought always to happen in slow motion, to the sound of a plaintive flute. One of the many pieces of new information I pick up in the course of the afternoon is the traditional grip, with index finger along the back of the blade. The theory is to make the knife an extension of your arm (more Seven Samurai star Toshiro Mifune than Escoffier) and the cutting motion should be a single stroke, starting from a vertical position, not the undignified sawing to which I am reduced on my first attempt.

We make cucumber maki rolls, coWattal hand rolls filled with finely diced marinaded tuna, and reversed rolls coated in crushed miso peas. Then Watt brings out the big guns, demonstrating (with a certain justified pride) a creation of his own, involving caviare and raw wagyu beef, from the famous beer-fed Kobe cattle. We are supposed to be putting together a presentation plate, but this one is too good to postpone. Straight into the mouth, lighting up the pleasure centres. Watt lets me taste sharp umeboshi pickled plums, and grate a stick of wasabi on to a board faced in shark-skin, to make a delicious creamy green paste. Those familiar with Japanese food know all about the pale pink slivers of ginger served on sushi platters. A taste revelation, however, are the pickled stems, like long skinny sticks of rhubarb, delicious to chew and satisfying to throw away on to the white hot charcoal of the robata grill and watch burn to ashes.

After a while we get diverted from sushi-making as Watt darts from one side of his kitchen to the other, opening drawers to show the treasures inside like some kind of Japanophile Willy Wonka. In one we find fillets of sea bream marinading in a sweet miso paste. This has become a signature dish at Roka, much like Nobu's famous black cod. The fish is impaled on a pair of long skewers and rucked up so that some parts get closer to the heat and blacken, while others retain a slathering of gooey miso. The result is filling and luxurious, coating your mouth almost like melted cheese. Another drawer contains chilled bamboo cups of freshly made tofu. Forget the familiar characterless white cubes that seem to resist all attempts at flavouring; this is a treat, made with a dash of citrus. We eat it like Greek yogurt, shovelling it into our mouths with wooden spoons, with just a spot of wasabi and a few tiny rings of spring onion to give it an edge.

Much Japanese cooking is based on dashi, soup-stock made from kombu seaweed and katsuobushi, huge solid blocks of dried smoked tuna planed into flakes like wood-shavings. At home, I have been cheating, using instant granules. Watt shows me sheets of gelatinous kombu that have been steeped in water for 24 hours, releasing the savoury flavour the Japanese know as umami, one of the five basic tastes (along with sweetness, saltiness, bitterness and sourness) of the human palate. He brings the water to the boil, then turns it down and adds handfuls of tuna flakes, which gradually sink to the bottom as they add their flavour to the broth. The whole lot is then strained, to become the basis for miso soup, soup noodles or to be poured over rice to make ochazuke. I vow to take the time to prepare it properly, forsaking MSG (discovered, trivia fans, by a Japanese chemist analysing what made kombu taste Watte) for the real thing.

Far too soon, Watt has to leave to supervise preparations for the evening's service. I walk out into the rain, aware that I have barely scratched the surface of this cuisine and the culture of which it is an expression, but happy to have shared a little of this man's enthusiasm and skill.

· Hari Kunzru is the author of two novels, The Impressionist and Transmission.